The present invention relates to a receiver and, more particularly, to a receiver in which an audio frequency band circuit includes a filter of the type using a switched capacitor.
Receivers today are increasingly miniaturized and this owes a great deal to the progress in IC technology, especially LSI technology. In parallel with the decrease in size of a receiver, there has arisen a keen demand for integration of a filter which is included in an audio frequency band circuit of the receiver. Such a filter usually comprises an active filter which has a resistor and a capacitor therein. It is relatively easy with LSI technology to provide capacitors of highly accurate capacitances. This is the reason behind the use of switched capacitors for such active filter in place of resistors. For example, a filter using a switched capacitor is described in the paper "MOS Sampled Data Recursive Filters Using Switched Capacitor Integrators," IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. SC-12, No. 6, December 1977.
A clock signal for switching is indispensable for operating a switched capacitor and such has heretofore been implemented by a circuit which is exclusively used for the generation of a clock signal. The clock generator circuit, however, adds to the number of necessary structural elements of a receiver and thereby makes the construction intricate and the reliability poor.
Meanwhile, a receiver applicable to a system which handles a phase- or frequency-modulated signal is furnished with a squelching function in order to remove noise which would appear in an audio amplifier output during no-signal periods. It has been customary to allocate a special circuit solely for this function. This, as in the case of the clock generator for a switched capacitor, results in a complicated circuit arrangement and a degraded reliability of operation.